Field of the Invention
The invention concerns techniques to determine a magnetic resonance image of an examination subject with at least two spin species (types) by means of at least two approximative models that express an MR signal under consideration of various MR parameters.
Description of the Prior Art
Chemical shift imaging multi-echo magnetic resonance (MR) measurement sequences (for example Dixon-like measurement sequences) are helpful in many applications. Such techniques make use of the effect that the resonance frequency of the nuclear spins depends on the molecular or chemical environment. This is known as a chemical shift. For example, the difference between two resonance frequencies can be expressed in ppm (“parts per million”, i.e. 10−6), and in particular independently of field strength. The chemical shift between nuclear spins in water and nuclear spins in fatty acid chains is frequently considered. Body fat is generally composed of multiple components with different chemical shifts, such that multiple resonance frequencies are obtained (what is known as the multispectral nature of fat). A spin species with a shift of approximately 3.4 ppm typically dominates. The MR signals of typical Dixon-like measurement sequences are therefore frequently reconstructed in a water MR image and a fat MR image, i.e. in individual MR images of the respective spin species.
Fat quantification by means of such multi-echo MR measurement sequences (and MR images obtained from these) is an increasingly important means in the diagnosis of the most varied illnesses, for instance fatty liver disease. A water image can serve as a replacement for conventional spectral fat suppression.
Inhomogeneities of the basic magnetic field of the MR system can also cause a shift of the resonance frequency, which can be at least partially compensated by special correction methods and/or more complicated multi-echo MR measurement sequences (for example with additional echoes). More comprehensive approximative models can also be used for signal analysis. Using additional MR signals, an estimate can also be obtained for the T2* relaxation time that manifests in an echo time-dependent reduction of the signal strength. If T2* is not taken into account, this can lead to a worsening of the water/fat separation. Conversely, the absence of consideration of the multispectral nature of the fat can lead to a worsening of the T2* estimation.
T2* can serve, for example, as a measure of an iron content, for example for diagnosis of hepatic iron storage illness. The same can similarly apply to additional MR parameters. MR images that describe T2* can, for example, be acquired within the scope of what is known as relaxometry; in general, MR images that describe MR parameters are acquired within the scope of parametric imaging.
Different MR measurement sequences can be used for data acquisition: in general, such techniques can be implemented with every MR measurement sequence. Gradient echo (GRE) measurement sequences have proven to be particularly suitable.
For example, a comprehensive signal model for MR signals given multiple spin species (for example fat and water) is known from M. Bydder et al. in Magn. Reson. Imaging 26 (2008) 347-359; see for example Equation 5 of this publication. An expression for the MR signal is expressed there under consideration of multiple MR parameters. The MR parameters therefore serve as model parameters. The MR parameters include the resonance spectrum of the various spin species, the relaxation times of the various spin species (thus in particular T1 and T2* relaxation times), inhomogeneities of the basic magnetic field etc., as well as the strength of the noise. Additional MR parameters can result from the measurement sequence that is used, for example the phase position of the individual echoes. The spin species themselves can be considered as MR parameters. Various approximative models are known to solve this equation. The approximative models express the MR signal in a simplified form relative to Equation 5. The MR parameters can thereby be considered only in part or, respectively, to a limited or, respectively, simplified extent. A quantitative determination of these MR parameters (in particular the clinically relevant fat/water quantification or T2* determination as presented above) is then possible by means of the multi-echo MR measurement sequence.
In general, the following trend can be formulated: the more precise a solution of the cited Equation 5 that is sought—for example in that more MR parameters are taken into account as open variables and/or that individual MR parameters are precisely taken into account (for example a larger number of resonance peaks and so forth)—the more MR signals that are required. Otherwise, an approximative model can deliver only limited stable solutions, and an uncertainty of the solution increases, which can produce an intensified noise of the MR image, for example.
It can be worthwhile to use a relatively large number of different MR parameters. For this, it can be necessary to detect a comparably large number of MR signals. For example, the detection of six echoes at various echo times is known from US 2009/0261823 A1.
The large number of detected MR signals can have the disadvantage that an extended measurement duration can produce movement artifacts. This can in turn increase the uncertainty or the errors of the determined solutions.
Techniques are also known that allow a simplified determination of different MR parameters by means of defined approximative models. For this purpose, more or less wide-ranging simplifications are typically assumed, for example a lower number of MR parameters affecting the MR signal, constant or simpler MR parameters (for example a lower number of resonance peaks or resonance frequencies to be considered) and so forth. The parameter space or the number of unknown MR parameters can be reduced, and it can then be possible to implement a determination of the remaining MR parameters from a comparably lower number of MR signals. For example, for fat/water separation two-point and three-point techniques are known, thus techniques which take into account only two or three MR signals. These can be detected in a smaller amount of time and are suitable for (for example) examinations of the abdomen (in particular of the liver) during a breath-hold phase.
However, such severely simplified techniques can have the disadvantage that the quantitative determination of the MR parameters (in particular of the spin species) is comparably imprecise. For example, an overestimation or underestimation of the fat content can result, which can produce errors in a subsequent clinical application.
Therefore, there is a need for improved techniques to determine an MR image with at least two spin species by means of a chemical shift imaging multi-echo MR measurement sequence. In particular, here there is a need for techniques which allow a fast and simultaneously precise determination of the MR image.